The present invention relates to floating lenses, and more particularly to a floating lens extending mechanism in which the front and rear lens systems are moved relative to each other for short distance aberration compensation.
In general, the performance of a lens is lowered when a close focusing distance is taken with the lens system extended. This phenomenon is significant especially with a macro lens which can make a close shot. In order to overcome this difficulty, there have been proposed a variety of floating lenses which are so designed that the lens system is divided into front and rear lens groups, and in association with the extension of the lens, these lens groups are moved relative to each other for compensating for the aberration, so that the performance of the lens is maintained satisfactorily even when a close shot is made.
In the floating lens, fundamentally the rear lens group (hereinafter referred to as a "rear group lens", for convenience in description) is moved straight back and forth by rotation of a helicoid ring similar to the ordinary lens, and the front lens group (hereinafter referred to as "a front group lens", for convenience in description) is moved straight back and forth by rotation of the front group helicoid ring which is rotatably supported on the rear group lens frame. Therefore, the rear group lens and the front group lens can be individually moved by rotation of the respective helicoid rings according to the leads thereof (and the thread pitches of the helicoid rings with respect to the supporting members thereof), and accordingly the close focusing distance compensation can be made by suitably setting the leads (and the thread pitches). On the other hand, the conventional lens is so designed that the front and rear group helicoid rings are made integral with each other with an engaging member so as to be turned by the distance ring. Accordingly a lens such as a macro lens, the amount of extension of which is relatively large, suffers from the following difficulty: In the case where the amount of extension is large, it is necessary to increase the length of the distance ring in order to maintain the distance ring engaged with the front (or rear) group helicoid ring, or to use double helicoid threads to extend the distance ring itself. However, the former method cannot satisfy the requirement for miniaturizing the lens because the total length of the lens is necessarily increased, and the latter method suffers from an operational difficulty in that the distance ring is displaced.
In the above-described conventional extending mechanism, the loci of the front and rear group lenses with respect to the angle of rotation of the distance ring are linear, and accordingly only linear compensation is carried out according to the leads (and the pitches). However, in many cases it is desirable that the close focusing distance compensation of a lens system be a non-linear one, and in the above-described conventional extending mechanism the non-linear close focusing distance compensation is merely replaced by a linear one approximated thereto.